Impaired production of blood cells is associated with morbidity and mortality. The classical model for blood cell replenishment is changing with the unexpected realization that long-term hematopoietic stem cells (LT-HSCs) directly sense and respond to pathogens via toll-like receptors (TLRs). Following TLR stimulation, LT-HSCs proliferate and preferentially undergo myeloid-specific differentiation. Direct sensing of TLR ligand is thought to enable HSCs to replenish innate immune cells that are rapidly depleted during infection. However, in contrast to the potential benefits of short-term HSC activation, chronic TLR stimulation dramatically impairs long-term HSC function. We recently showed that murine HSCs chronically exposed to low-dose TLR4 agonist in vivo hyperproliferate, exhibit myeloid skewing, and fail to self-renew. These observations have important implications because TLR4 can be activated by plasma lipopolysaccharide (LPS) present in patients with chronic infection as well as by both LPS and dietary lipids that are chronically elevated in patients suffering from obesity. However, the cellular and molecular mechanisms underlying TLR4-mediated HSC exhaustion remain largely unknown. In Aim 1, we determine the role of inflammatory cytokines, and the cellular source of TLR4 sensing, in each a defined experimental model of chronic LPS-mediated HSC skewing and in the disease model of obesity. In Aim 2, we examine the molecular mechanisms underlying HSC skewing. These studies will be the first to establish the cellular and molecular mechanisms by which TLR stimulation impairs HSC function, and will open new areas of investigation into the mechanisms that preserve HSC integrity in health and disease.